Metal vapor discharge lamps are employed where high brightness is desired or required, such as for projectors, and theater and studio lighting. Most of these lamps are double ended; i.e., the electrodes extend along a single longitudinal axis, and/or they are D.C. operated. Single ended arc lamps are known but suffer from high open circuit voltages necessitating massive ballasts, blackening of the arc chamber in the stable arc region and long transition times from full glow state to arc, thus allowing more time for sputtering and blackening to occur.